1. Field of the Invention
The present invention relates to lithium batteries and to a method for forming a metal anode protective layer for lithium batteries. More particularly, the present invention is directed to a method of forming a LiF protective layer on a lithium metal anode surface with enhanced adhesion, improved interfacial stability due to suppression of dendrite growth on the anode surface, and with extended lifetime due to the improved energy density and cycling characteristics.
2. Description of the Related Art
As the weight of portable electronic devices, such as camcoders, mobile phones, and notebook PCs, becomes lighter and as the level of diversified functions of such portable electronic devices becomes greater, research on batteries as driving power sources is increasing. In particular, rechargeable lithium secondary batterys have received the greatest amount of attention for its fast charging rate and a weight-per-energy density that is three times higher than conventional lead storage batteries, nickel-cadmium batteries, nickel-hydrogen batteries, and nickel-zinc batteries.
When the anode of a lithium secondary battery is formed of lithium metal, the lithium metal reacts with electrolytes, water, organic solvents, or lithium salts and thus forms a solid electrolyte interphase (SEI). Such SEI leads a localized current density difference and facilitates the growth of dendrites through reactions with the lithium metal during charging. The dendrites grow larger and larger as charging-discharging cycles are repeated and eventually lead to electrical shorts between the cathode and the anode. Dendrites are mechanically unstable because of their bottle necks, and thus form a dead lithium that not only decreases the capacity of the lithium metal anode but also reduces the safety of the battery due to a high surface area of a dead lithium. As a result, battery capacity and cycling lifetime decrease with poor battery stability.
To overcome the foregoing problems, a feature of an embodiment of the present invention provides a method for forming a lithium metal anode protective layer capable of suppressing the growth of dendrites on the anode.
Another feature of an embodiment of the present invention provides a lithium metal anode employing a protective layer having enhanced interfacial stability between the lithium metal anode and the electrolyte.
Yet another feature of an embodiment of the present invention provides a lithium battery comprising a lithium metal anode having improved energy density and extended lifetime due to enhanced cycling characteristics.
In accordance with a first preferred embodiment of the present invention, there is provided a method for forming a lithium metal anode protective layer for a lithium battery having a cathode, an electrolyte, and a lithium metal anode sequentially stacked with a lithium metal anode protective layer between the electrolyte and the lithium metal anode, comprising activating the surface of the lithium metal anode and forming a LiF protective layer on the activated surface of the lithium metal anode.
The surface of the lithium metal anode is preferably activated before forming the protective layer. Preferred methods to activate the lithium metal anode includes mechanical etching, chemical etching, electrochemical etching, and plasma etching. Suitable mechanical etching methods include common etching techniques, such as polishing, grinding, and lapping as well as a scratching using a scratching device such as a Nylon brush. As the surface of the lithium metal anode is activated, impurities and solid electrolyte interphase can be removed from the lithium metal anode surface. The reactive surface area of lithium with respect to the polymeric protective layer increases so that reactivity therebetween is improved.
The LiF protective layer may be formed using a fluorine-containing polymeric layer formed on the activated surface of the lithium metal anode. The LiF protective layer may also be formed by subjecting the lithium metal anode to a fluorine-containing gas atmosphere, for example, CF4 or C2F6.
While any polymer may be applied for the fluorine-containing polymeric layer so long as it contains fluorine, polytetrafluoroethylene, polyvinylidene fluoride, vinylidenefluoride (VDF)-hexafluoropropylene (HFP) copolymer, polytetrafluoroethylene-hexafluoropropylene copolymer, polychlorotrifluoroethylene, perfloroalkoxy copolymer, and fluorinated cyclic ether are preferred.
An inorganic filler such as zeolite, fumed silica, titanium dioxide, and aluminium oxide may preferably be added to improve the mechanical strength of the polymeric protective layer and thus suppress the growth of dendrites.
In accordance with a second preferred embodiment of the present invention, there is provided a method for forming a lithium metal anode protective layer for a lithium battery having a cathode, an electrolyte, and a lithium metal anode sequentially stacked with the lithium metal anode protective layer between the electrolyte and the lithium metal anode, comprising activating the surface of the lithium metal anode, forming a fluorine-containing polymeric layer on a separator and coating an inorganic filler dispersion solution on the fluorine-containing polymeric layer to form a composite layer of inorganic filler layer/fluorine-containing polymeric layer/separator, and applying the composite layer of inorganic filler layer/fluorine-containing polymeric layer/separator on the activated surface of the lithium metal anode to form the lithium metal anode protective layer on the lithium metal anode.
In this second preferred embodiment of the present invention, the surface of the lithium metal anode can be activated using the same methods described above in connection with the first preferred embodiment of the present invention. Since the inorganic filler layer is interposed between the lithium metal anode and the fluorine-containing polymeric layer, degradation of the fluorine-containing polymeric layer resulting from repeated charging-discharging cycles may be prevented, and the interfacial stability of lithium metal is improved. Alternatively, multiple inorganic filler layers may be formed through repeated depositions.
In accordance with a third preferred embodiment of the present invention, there is provided a lithium metal anode having a protective layer formed by one of the methods described above.
In accordance with yet a third preferred embodiment of the present invention, there is provided a lithium battery comprising an anode protected by a protective layer formed by one of the methods described above.